Category: Extreme Events

A storm predicted to make landfall over northern California on 5 March 2016 has characteristics of an Atmospheric River. The AR is showing moderate strength with an average landfall duration of 12-24 hours. The storm has the potential for R-Cat 1 rainfall in favored mountain areas. For up to date AR forecasts visit the CW3E AR Portal.

CW3E gives an outlook on current and upcoming Atmospheric River (AR) events along the U.S. West Coast. A moderate AR is currently impacting central California producing light precipitation over the northern Sierra Nevada and Central Valley. Two ARs are expected to make landfall over the Pacific Northwest in the next seven days, with both propagating south over California prior to dissipation. Long range forecasts also show the potential for another strong AR to make landfall in the Pacific Northwest next week.

El Nino-fueled storms over the last few days, including a land-falling atmospheric river on 5 January (see cw3e.ucsd.edu for more details), have brought San Diego county’s accumulated winter precipitation to well above normal for this time of year. These numbers are tracked by the California-Nevada Applications program (cnap.ucsd.edu) and the Center for Western Weather and Water Extremes (cw3e.ucsd.edu) at UCSD’s Scripps Institution of Oceanography, in association with KPBS. The precipitation tracking tool shows the impact of the recent storms on the accumulated winter precipitation:

In the last three days, San Diego county has received almost 17% of the amount of precipitation it receives in an entire year, on average.

Precipitation gauge records compiled by the California Nevada River Forecast Center show accumulations of 3-5″ over much of the Los Angeles basin in the 72 hour period, reaching 7″ in some locations in the surrounding mountains. San Diego county accumulations are 1-3″ over much of the city, and 4-5″ in the North County interior:

These storms bring San Diego county’s running total accumulated precipitation so far this winter to 42% of the total typically received over an entire average year. This is an above-average value for the first time since the winter started; typically, by this point in the winter, San Diego will have accumulated 32% of the average year-end total precipitation.

Before the recent storms hit, the accumulated precipitation in the Los Angeles region since the winter started was only 11% of the typical end-of-winter total, far below the average value of 29%. In three days the accumulation has jumped to 27%. This rapid increase is more than is experienced in 95 out of 100 wet periods of 3-day duration in the region, leading to strong flows in the normally quiescent Los Angeles river and localized flooding in Southern California coastal areas.

Precipitation during the week was widespread, with rain and snow delivered from Washington to Northern Mexico. California statewide totals also were boosted during the Jan 4-7 period (cnap.ucsd.edu), although amounts were not as heavy in Northern California as they were in Southern California.

These accumulated precipitation totals can be tracked in real time from the California-Nevada Applications Project (CNAP) and Center for Western Weather and Water Extremes (CW3E)’s “San Diego Precipitation Page”. This includes monitoring of how current winter precipitation compares to that during the last 5 strongest El Nino winters (1982-83, 1997-98, 1957-58, 1972-73, and 1965-66), shown as the colored lines in the upper panels:

The current El Nino event is is one of the three strongest in records that go back to 1950, with a broad area of unusually warm ocean surface waters in the tropical Pacific ocean and highly anomalous winds and other atmospheric conditions in the region. El Nino events can alter the North Pacific winter storm track, producing unusually wet winters in Southern California about 60% of the time, compared to about 11% of the time during years when no El Nino or its opposite, La Nina, is present. Forecasts indicate that El Nino conditions are likely to persist through the spring of 2016, before fading in the early summer of 2016.

Points of contact: David Pierce (dpierce@ucsd.edu), Dan Cayan (dcayan@ucsd.edu), and Marty Ralph (mralph@ucsd.edu) at the Scripps Institution of Oceanography, UCSD.

Pacific Northwest Storm of 13-15 November 2015: A Synopsis of Landfalling Atmospheric River Conditions

November 25, 2015

CW3E researcher Brian Kawzenuk provides an analysis and synopsis of an Atmospheric River that made landfall along the U.S. Pacific Northwest over the 13-15 November 2015 period. The AR made initial landfall along the Washington coast and lead to significant precipitation for nearly three days throughout western Oregon, Washington, and British Columbia. The AR initially developed near Japan and propagated across the entire North Pacific Ocean before making landfall.

Above is a sequence of 30-minute NEXRAD radar composite imagery from 12-15 November 2015 which shows precipitation throughout the Pacific Northwest during nearly the entire period.

The above loop shows SSMI Integrated Water Vapor during 10-15 November 2015.

A Preliminary Summary of Highway 58 and I-5 Flooding Event of October 15, 2015

October 27, 2015

Nina Oakley (WRCC/DRI), Jeremy Lancaster (CGS), John Stock (USGS), Brian Kawzenuk (CW3E), and Mike Kaplan (DRI) provide an analysis and synopsis of the meteorological and geological conditions that produced alluvial fan flooding over portions of Highway 58 and Interstate 5 in southern California. A weakening cutoff low that had entrained subtropical moisture moved onshore over southern California, initiating convection and localized heavy precipitation. Hillslope runoff concentrated in steep valleys where it entrained debris. The debris then flowed onto steep alluvial fans at the base of these valleys, inundating portions of I-5 and State Hwy 58.

The Center for Western Weather and Water Extremes (CW3E) is grateful for the approval of legislation that will improve California’s ability to respond to major precipitation episodes. This legislation, recently approved, will aim to allow the state of California to better manage water supplies by expanding climate and weather research that is focused on the causes of drought and flood.

The two images below show an example of research aimed at improving forecasting ability. The two maps show the integrated water vapor (IWV) forecast from February 9, 2014. The top panel shows a CW3E simulation by a regional model (called West-WRF). The bottom panel shows a national forecast by the Global Forecasting System (GFS). The CW3E simulation offers a resolution of 9km while the national forecast is at 0.5 degrees (approximately 100km). This improved model forecast horizontal resolution will allow forecasters to better pinpoint heavy precipitation events aimed at the west coast.

California Storm of 5 January 2016: A Preliminary Synopsis of a Marginal Landfalling Atmospheric River

July 24, 2015

CW3E researcher Brian Kawzenuk provides an analyis and synopsis of an extreme precipitation event over the Southwestern United States during the 18-20 July 2015 period. Former Hurricane Dolores provided high amounts of atmospheric moisture to the Southwestern United States with allowed for multiple showers and thunderstorms to develop on 18 and 19 July 2015. Monthly precipitation records were broken in 48 hours throughout Southern California, which caused multiple landslides and flash floods.

California Storm of 10-12 December 2014: A Synopsis Including Landfalling Atmospheric River Conditions

July 10, 2015

CW3E researcher Brian Kawzenuk provides an analysis and synopsis of an Atmospheric River that made landfall along the U.S. West Coast over the 10-12 December 2014 period. The AR made initial landfall along the Oregon coast and propagated south before dissipating over southern California. Up to 350 mm of 72-hour precipitation was produced in northern California representing up to 45% of total water year to date precipitation. The precipitation from this event provided many drought-stricken California reservoirs with significant amounts of water supply and improved drought conditions throughout northern California.

Above is a sequence of 30-minute NEXRAD radar composite imagery from 10-13 December 2014 which shows the penetration of the heaviest precipitation.

The above loop shows the strong atmospheric river making landfall and the associated integrated water vapor (color bar in cm).

CW3E researcher Nina Oakley provides a preliminary synopsis for a closed low that developed in the Pacific Northwest and moved south along the California/Nevada border over the 6-10 May period. The closed low turned east over southern California with a very cold core moving over a warm surface. The easterly flow produced orographic precipitation on the eastern side of the Sierra with minimal precipitation on the western side of the Sierra. Large areas of the Great Basin experienced precipitation amounts that were not extreme but significant for May.

The origins of extreme precipitation events in the Western U.S. range from landfalling atmospheric rivers, to the summer monsoon, upslope storms on the Rocky Mountain Front Range, and deep convection of the Great Plains variety. This was shown by an analysis across the west of the seasonality of the top 10 wettest days for each of thousands of COOP observer sites (Ralph et al. 2014**). Each of these sites had at least ~10,000 data points, so these top 10 days represent roughly the top 0.1% of days. Some areas were universally dominated by events in one season, or two. A couple of areas stood out in the diversity of their seasonality of extreme daily precipitation, including Colorado.

The study presented in Mahoney et al. 2015* explores this local variability more deeply, explores how the devastating flood of September 2013 in Colorado’s northern Front Range is related, and describes some of the implications of the findings for flood control and other sensitive sectors. The co-authors represent a diverse group themselves, including climate, weather, hydrology, hydrometeorology expertise from several organizations, (CIRES, NOAA/PSD, Scripps/CW3E and CSU). The paper is highlighted here as it represents an example of work on extreme events in the Western U.S. that the Center for Western Weather and Water Extremes is contributing to.

Abstract of Mahoney et al. 2015*: The climatology of Colorado’s historical extreme precipitation events shows a remarkable degree of seasonal and regional variability. Analysis of the largest historical daily-precipitation totals at COOP stations across Colorado by season indicates that the largest recorded daily precipitation totals have ranged from less than 60 mm/day in some areas to greater than 250 mm/day in others. East of the Continental Divide winter events are rarely among the top 10 events at a given site, but spring events dominate in and near the foothills; summer events are most common across the lower-elevation eastern plains, while fall events are most typical for the lower elevations west of the Divide. The seasonal signal in Colorado’s central mountains is complex; high-elevation intense precipitation events have occurred in all months of the year, including summer when precipitation is more likely to be liquid (as opposed to snow) which poses more of an instantaneous flood risk.